Volcanic Caves of Western Victoria Ken G
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Volcanic Caves of Western Victoria Ken G. Grimes RRN 795 Morgiana Rd., Hamilton, 3300, Australia ([email protected]). Abstract The Western District Volcanic Province extends from Melbourne across to the Mount Gambier area and has been erupting basalt lavas for at least the last 5 million years. Lava caves have formed in several areas across the region, but the best concentrations are in the ~30,000 year-old lavas from Mt. Eccles and Mt. Napier. There are a variety of volcanic caves, including large feeder tubes that are responsible for the long lava flows (60 km in the case of a flow from Mt Rouse), but also smaller but more complex shallow subcrustal lava caves and one example of a still-open volcanic vent or large hornito. Lava tubes form in two main ways. The first is by the roofing of narrow surface lava channels, which happens in several ways. This type tends to form linear and simply-branching or anastomosing tubes. The second way is by draining from beneath the crust of a set of spreading lava lobes near the leading edge of a lava flow - these tend to form more complex mazes of shallow, low-roofed chambers and passages, but over time they may evolve by solidification of the more stagnant areas and erosional enlargement of the fastest moving routes to form simpler linear tubes that are difficult to distinguish from the roofed channels. Both types of tube contain liquid lava flowing beneath a solid crust. At the end of the eruption some of that lava drains out to leave empty caves, but most tubes remain filled with solidified lava. Many lava caves end at solid undrained lava “sumps”. The Western District Volcanic Province The Western District Volcanic Province of western Victoria (previously known as the Newer Volcanics Province) is one of the world’s larger volcanic plains, and has formed by a succession of eruptions and basaltic lava flows over the last five million years. The isolated volcanoes near Mount Gambier are a western outlier of the Province (Figure 1). Eruptions have continued up to quite recent times and further eruptions could occur in the geological future. Current dating suggests that the youngest volcano may be Mount Schank, south of Mount Gambier, which erupted 5,000 years ago. The flows associated with these younger eruptions show better lava caves and surface features than Figure 1. those of the older volcanics. None-the-less, a Western District Volcanic Province & caves. few of the caves are in flows several million years old. Lava tubes and other volcanic caves are scattered across the province (Figure 1), but the majority of them are in the western area where they are associated with two of the younger eruptions in the region - Mt Eccles and Mt Napier (Webb & others, 1982, Grimes & Watson, 1995, Grimes, 2008). 10 p1 Volcanic Caves of Western Victoria Ken G. Grimes Surface landforms The volcanics are dominantly built up from basalt lava flows, but there are numerous small volcanic cones built by explosive activity, as well as larger maar lakes formed by major explosions (Price & others, 2003; Joyce & Webb, 2003). The older volcanoes of the region have degraded features, and thick lateritised soils, which make their recognition difficult. By contrast, the flows from the younger eruptions have only minimal soil development and rough undulating surfaces known as stony rises. Isotope dating suggests that these are all less than 500,000 years old. The best modern model for the nature of vulcanism in this region is provided by the Hawaiian volcanoes. There we see broad lava shields built up by successive flows of very fluid basaltic lava spreading out from a central crater or fissure. In the crater area we see lava pools with fountains jetting into the sky and building local small cones of welded spatter or loose scoria. The long lava flows are seen to be fed either by surface channels, or underground by lava tubes. Local examples of lava shields are the lower slopes of Mount Napier and the lava fields surrounding Mount Eccles. However, in Victoria we also have slightly more explosive eruptions which build larger scoria cones; and the maar lakes (eg. Tower Hill), which are large but shallow craters formed by major steam-driven explosions where rising magma intersected water- saturated limestone. At Mount Eccles a line of scoria cones running southeast from the main crater could have formed along a fissure eruption. Lava flows: Basaltic lava is a hot (1100°C) liquid that can flow readily. There are two main forms of basaltic lava flow, which grade into each other. Pahoehoe lava is the most liquid form - characterised by the formation of thin smooth skins that become wrinkled (hence its alternative name of ‘ropy lava’). Pahoehoe lavas advance as a succession of lobes, each of which develops a skin, is inflated by the liquid pressure within, then ruptures at one or more points to release liquid lava to form new lobes (Figure 4). As pahoehoe loses gas and cools it becomes frothy and stiffer. The surface tends to crack, twist and break into angular, often spiny, blocks to form what is called aa or ‘blocky’ lava. Behind the advancing lava front solidification of stagnant areas restricts lava movement either to narrow surface channels, or internally in lava tubes beneath a surface crust. Overflow from the surface channels builds up levee banks of thin sheets or spatter. Larger flows across a levee can feed lateral lava lobes with small internal lava tubes. A major breach of a levee may result in a large side flow, fed by its own channel, and the original channel may be abandoned. Good examples of lava channels (locally referred to as ‘canals’) occur at Mount Eccles. A number of shallow lava tubes are known in flows that have run off to the sides from these channels (Figure 14; Grimes, 1995 & 2008). Lava tubes provide good insulation for the hot lava flowing within them. This allows the formation of very long flows such as the 50km Tyrendarra Flow from Mount Eccles, which extends offshore across the continental shelf (which was dry at the time), and the older 60km flow from Mount Rouse, which may also extend offshore (Figure 2). When a lava flow follows a valley, as in the Harman Valley flow from Mt. Napier and the Tyrendarra flow from Mount Eccles, it disrupts the drainage. Twin lateral streams may run down each side of the original valley. Swamps or lakes will form where the flow enters the valley, and where tributary valleys have been dammed by the flow. 10 p2 Volcanic Caves of Western Victoria Ken G. Grimes Formation of Volcanic Caves Lava tubes form in basaltic lava flows by two main processes (Peterson & others, 1994; Halliday, 2004)> One is by the roofing over of surface lava channels. Figure 3 gives details of the three ways of doing this. Later overflows through sky-lights may thicken the tube's roof from above. On many cases linings plastered on the walls, or collapse modifications, make it hard to distinguish the three modes. The other way of making lava caves by the draining of still molten material from beneath the solidified crust of a flow. Figure 4 illustrates the formation of these subcrustal lava caves. In its simplest form, drainage of lava from beneath high areas on the crusted surface will form simple isolated chambers. Complex nests of advancing lava lobes create equally complex patterns of active tubes and chambers which can later drain to form open caves (Figure 4, steps 1, 2 & 3a). Lava lobes can be stacked vertically as well as advance forwards so that a complex three- dimensional pattern of branching tubes can form. As lava continues to flow through these complex systems they will evolve by erosion and solidification to form larger, more streamlined, linear tube systems that act as “feeder tubes” to carry hot lava to the advancing lava front (Figure 4, steps 3b & 4). If sufficiently evolved, these linear tubes can converge on the form of the, generally larger, linear tubes formed by roofing of surface lava channels. Thus the genesis of many large lava caves remains difficult to deduce. The long lava flows in the region would all have been fed by large cylindrical lava tubes; but only parts of these would have drained at the end of the eruption to form open caves. Many lava caves end at solid undrained lava “sumps". Features found in Volcanic Caves The lava caves contain a distinctive suite of lava structures or “decorations”, some of which are illustrated in Figures 7-11. The level of lava within the tubes tends to fluctuate during the course of the eruption, and so we find thin linings plastered onto the walls and roofs, and ‘tide-marks’ are indicated by solidified linings, benches or shelves on the sides of the tubes (Figure 11). Some shelves can reach right across a passage to form a false floor (Figure 8). The thin wall linings can rupture, peel back and curve over to form draperies and scrolls. Some linings are smooth, but others have a sharp hackly surface which may be due to the bursting of many small gas bubbles (Figure 10). Rafted slabs floating on a flow surface may leave grooves and striations on the semi-solid wall linings. Lava “hands” of semi-solid lava can be squeezed out through cracks or holes in the lining. Small round-tipped lava stalactites, (lavacicles, lava drips) form where molten lava has dripped from the roof (Figure 10). Lava ribs form where lava dribbled down the walls of the cave, or where the whole lining has sagged and wrinkled.